In an exhaust gas purifying system which is equipped with an NOx occlusion reduction type catalyst (“NOx catalyst”) and an oxidation catalyst set on the upstream side of the NOx catalyst, and in which sulfur purge control for regenerating the sulfur-poisoned catalyst is conducted, an air fuel ratio control for sulfur purge is conducted when a temperature difference between an inlet side and an outlet side of the NOx catalyst is outside a given range. Sulfur poison on the NOx catalyst can be uniformly removed in sulfur-purge regeneration control, thereby extending the service life of the NOx catalyst. Further, the NOx catalyst can be protected from thermal deterioration caused by partial temperature rise inside the NOx catalyst on its downstream side, so that the service life of the NOx catalyst can also be extended.